Transistors are commonly used as the switching device in circuits supplying a load which requires a switching source, such as power supplies, motor drivers, amplifiers, etc. Such circuits may utilize one or more transistors of various types, such as Metal Oxide Field Effect Transistors (MOSFETs, or simply FETs) or Insulated Gate Bipolar Transistors (IGBTs), arranged in various topologies. The voltage across the transistor may be fixed or unknown, depending on the particular topology.
The control voltage necessary to drive a particular switch is a function of the switch type and topology. For example, the control, or gate voltage necessary to drive a high current, power FET must present a predetermined differential voltage across the gate and source terminals of the FET, such as on the order of 10 volts. More particularly, in an N-channel FET, the gate voltage must be brought to approximately 10 volts higher than the source voltage and in a P-channel FET, the gate voltage must be brought to approximately 10 volts lower than the source voltage. The absolute value of the control voltage may be as low as 10 volts in the case of driving a xe2x80x9clow sidexe2x80x9d FET in which the source terminal is coupled to ground or may be an unknown voltage in the case of driving a xe2x80x9chigh sidexe2x80x9d FET or floating low side FET in which the source terminal is at an unknown voltage.
Various circuits are used to provide switch control voltage. One such circuit is referred to as a xe2x80x9cbootstrapxe2x80x9d circuit and is most commonly used to drive FETs in applications in which the source voltage is unknown. The voltage across a bootstrap capacitor coupled between an input voltage source and the source terminal of the FET rises as the source voltage rises and thus, provides a voltage which is at a predetermined level higher than the source voltage. However, since the bootstrap circuit does not provide any isolation between the bootstrap voltage and the input voltage source, current and voltage transients caused by the switching FET can affect the input voltage source.
A charge pump, which generates an output voltage in response to a lower input voltage, is sometimes used to provide the necessary FET gate drive voltage. However, like the bootstrap circuit, the charge pump does not provide any isolation between the input voltage source and the power switch.
Pulse transformers can be used to provide isolation between the input voltage source and the power switch, in order to prevent transients from affecting the input voltage source. However, pulse transformers can be expensive and bulky.
It is an object of the present invention to provide a circuit for generating a control voltage for switches arranged in various configurations.
It is a further object to provide a circuit for generating a switch control voltage which provides isolation between an input voltage source and the switch without the use of a transformer.
These and other objects of the invention are achieved with a circuit including a capacitor, a first pair of switches adapted to coupled the capacitor to an input voltage source during a first time interval, and a second pair of switches adapted to couple the capacitor to a control terminal of a switch during a second time interval which is nonoverlapping with respect to the first time interval. The voltage across the capacitor provides a predetermined differential voltage which is independent of the input voltage or any other fixed voltage.
With this arrangement, the circuit can be used to provide drive switches arranged in various configurations, including high side switches and floating low side switches, in which cases a terminal of the switch is at an unknown, or floating voltage. Also, the above-described circuit provides isolation between the input voltage source and the switch, thereby advantageously preventing current and voltage transients from affecting the input voltage source.
A control circuit provides a first control signal to each of the first pair of switches in response to the voltage across the capacitor and a second control signal to each of the second pair of switches in response to the voltage across the switch. In one embodiment, a first comparator has a first input terminal coupled to the capacitor, a second input terminal coupled to a threshold voltage, and an output terminal at which an output signal is provided to the control circuit. The output signal is indicative of whether or not the voltage across the capacitor is greater than a predetermined level and the first control signal causes the first pair of switches to open when the voltage across the capacitor is greater than the predetermined level. In this way, the first pair of switches is opened to prevent the capacitor from charging to a voltage that is too high to safely drive the switch.
A second comparator has a first input terminal coupled to the control terminal of the switch, a second input terminal coupled to the reference terminal of the switch, and an output terminal coupled to the control circuit at which an output signal is provided. The output signal of the second comparator is indicative of whether or not the voltage across the switch is less than a predetermined level and the second control signal provided by the control circuit causes the second pair of switches to open when the voltage across the control and reference terminals of the switch is less than the predetermined level. In this way, charge on the capacitor is replenished to keep the capacitor voltage above a minimum level necessary to fully enhance the switch.